In accordance with a rapid spread of information relevant apparatuses, communication apparatuses and the like such as a personal computer, a video camera and a portable telephone in recent years, the development of a battery to be utilized as a power source thereof has been emphasized. The development of a high-output and high-capacity battery for an electric automobile or a hybrid automobile has been advanced also in the automobile industry and the like. A lithium battery has been presently noticed from the viewpoint of a high energy density among various kinds of batteries.
Liquid electrolyte having a flammable organic solvent as a solvent is used for a presently commercialized lithium battery, so that the installation of a safety device for inhibiting temperature rise during a short circuit and the improvement in structure and material for preventing the short circuit are necessary therefor. On the contrary, a lithium battery all-solidified by replacing the liquid electrolyte with a solid electrolyte layer is conceived to intend the simplification of the safety device and be excellent in production cost and productivity for the reason that the flammable organic solvent is not used in the battery.
The intention of improving performance of an all solid state battery while noticing an interface between a cathode active substance and a solid electrolyte material has been conventionally attempted in the field of such an all solid state battery. For example, it is known that interface resistance between a cathode active substance and a solid electrolyte material is reduced by covering the surface of a cathode active substance with LiNbO3. However, the covering of the surface of a cathode active substance with LiNbO3 allows the interface resistance between a cathode active substance and a solid electrolyte material to be reduced at the initial stage; yet, the problem is that the interface resistance increases with time. On the contrary, for example, in Patent Literature 1, an all solid state battery, in which a cathode active substance whose surface is covered with a reaction inhibition portion comprising a polyanionic structure—containing compound is used, is disclosed. This intends to achieve higher durability of a battery by covering the surface of the cathode active substance with the compound having a polyanionic structural part high in electrochemical stability to inhibit interface resistance between the cathode active substance and a solid electrolyte material from increasing with time. On the other hand, in Non Patent Literature 1, a polymer battery, in which LiCoO2 (a cathode active substance) whose surface is covered with Li3PO4 is used, is disclosed. This intends to achieve higher output and higher capacity of a battery by covering the surface of LiCoO2 with Li3PO4 to inhibit oxidative destruction of a solid polyelectrolyte caused at an interface between LiCoO2 and a solid polyelectrolyte.